The land mobile radio service has been in existence for some decades. With few exceptions, the service has been restricted to voice communication via frequency-modulation channels. Current practice is to use a central base-station in a given service area, transmitting sufficient power to cover the entire area with adequate field strength. In some cases, satellite stations transmitting the same message may be used to boost signal strength in hard-to-reach areas (the practice known as "simulcasting").
The current technology has the following limitations; (1) Large transmitted powers are needed to provide adequate coverage in radio "shadow" areas. The resulting field strength pattern is several orders of magnitude more intense than it need be over a large portion of the service area;
(2) Because of the high power required, each radio channel may be used once only over a wide geographical area. This is a highly inefficient use of the available spectrum, and makes necessary increasing use of spectral space as the demand for the service grows; and
(3) The quality of speech is poor because of the phenomenon known as Rayleigh Fading. The subjective effect is of repeated submersion of the required signal in background noise, frequently many times per second if the mobile unit is in motion.
Much recent technical literature has been addressed to the use of a cellular structure as a means of increasing the efficiency of the land-mobile radio service. The proposal is to divide the service area into cells (most often hexagons), each with a base station at its center. The communication link is completed by land line, with radio channels bridging the small gap between each mobile and its nearest base station. If a mobile moves to a new cell, the radio link is switched to the new base station.
The advantages claimed for the cellular/FM LMR system are as follows:
(1) Relatively low transmitted powers are required because each base station need only provide adequate field strength within its own cell;
(2) Each channel may be used more than once, provided this is done in cells which are separated by a sufficient distance. Hence the total set of available channels is divided into subsets, each assigned to a widely distributed group of cells. Adjoining and other nearby cells are all assigned different sets of channels, so that cochannel interference is controlled by the geographic pattern of channel assignments (this strategy results in increased efficiency in the system's use of spectral space); and
(3) The amount of cochannel interference is largely determined by the pattern of channel assignments, and not by cell size. Hence increased demand for service can be accommodated by dividing cells into smaller units.
The limitations and disadvantages of the proposed cellular/FM system are as follows:
(1) The effect of Rayleigh Fading upon signal quality is even worse than in existing systems, because background noise is augmented by cochannel interference;
(2) There is an inverse relationship between the number of channels per MHz which can be assigned to one cell and the resultant speech quality. The trade-off can be improved only by employing space-diversity techniques, which involve combining the signals from more than one antenna. But these techniques add significant complexity to the mobile units, and are very awkward to implement on hand-held mobile units;
(3) As the mobile moves from cell to cell, it is necessary to assign a new channel frequency to it. Thus, the central processor is required to keep track of the location of each mobile unit and of the assignment of each channel, and to issue instructions for changing channels via a dedicated control channel. This adds immensely to the complexity of both central plant and mobile units;
(4) If the mobile moves into a cell in which all assigned channels are occupied, its call will be terminated;
(5) FM channels are unsuitable for the reliable transmission of digital data. Hence the proposed cellular/FM system lacks flexibility with respect to possible new applications; and
(6) Because each user has essentially the same apparatus, eavesdropping is possible and message privacy is lost.
Hence, while mobile radio service is now provided, improvements therein have been needed to enable greater satisfactory use of such service, particularly in providing adequate area coverage and in providing more efficient use of the available frequency spectrum. In addition, while improvements such as cellular communications within a service area have heretofore been proposed, such proposals are not fully satisfactory in solving many of the problems presented and/or create new problems.